Adhesive compositions based on styrenic block copolymers as thermoplastic elastomer components are well known in the art. These compositions are, for instance, used as pressure sensitive adhesives (PSA) for industrial tapes, packaging tapes and labels, and in multipurpose hot-melt adhesive compositions which may be used to bond or construct articles in the manufacture of disposable soft goods, such as diapers, feminine care articles, surgical drapes and the like.
Poly(styrene)-poly(isoprene)-poly(styrene) block copolymers (S-I-S) and poly(styrene)-poly(butadiene)-poly(styrene) block copolymers (S-B-S) are widely used in these adhesive compositions. Both classes of block copolymers give the adhesive compositions specific properties related to the respective inherent characteristics of these block copolymers. For example, the softness of S-I-S makes this polymer the material of choice for pressure sensitive applications in tapes and labels. Alternatively, the elevated cohesion of S-B-S makes this material attractive for construction adhesives for disposable soft goods.
When compounded into hot melt adhesives, S-I-S polymers degrade by a chain scission mechanism; molecular weight is reduced and the cohesive strength of the adhesive is lowered. S-B-S polymers, on the other hand, tend to degrade by further chemical cross-linking increasing the cohesive strength of the adhesive, but also increasing the elastic modulus, forming a too hard and non-tacky adhesive. The thermal decomposition of both S-I-S and S-B-S based adhesives can ruin the utility of the adhesive product. It would be an advantage to the hot melt adhesive industry if polymers with less tendency to either fall apart (scission) or cross-link would be developed. In EP-669350 and U.S. Pat. No. 5,583,182, adhesive compositions have been described wherein the styrenic block copolymer is a block copolymer of an S-B-I-S type, an (S-B-I)n-X type or an (S-I-B)n-X type, wherein S represents a polystyrene block, B represents a polybutadiene block and I represents an isoprene block. These copolymers with block copolymer midblocks “B-I” combine some of the characteristics of S-I-S and S-B-S type polymers. However, the process to make blocks of polydienes in the midblock is demanding, more complex and requires long polymerization times. It has also been demonstrated that because of the blocky structure in the midblock, their heat stability performance is still inadequate and that the hot-melt viscosity of formulated adhesives is too elevated for industrial applications compared to respectively S-I-S or S-B-S based formulations.
In WO 00/14170 an adhesive composition is described based on an elastomeric component comprising (I) an SIS block copolymer and (II) an SBS block copolymer, and on a tackifying component comprising (III) a first hydrocarbon resin compatible with said SIS block copolymer and (IV) a second hydrocarbon resin compatible with said SBS block copolymer. However, the necessity to use two block copolymers (SIS and SBS) as well as two hydrocarbon resins is not a cost effective approach for the manufacture of adhesive compositions, as it requires the adhesive manufacturers to invest deeply in expensive feeding and dosing systems on their extruders.
In DE-2942128 an adhesive composition is described based on 100 part of a non hydrogenated block copolymer A-B-A where A is polystyrene and B a block made of a mixture of Butadiene and Isoprene; from 25 to 300 part of a tackifying resin; from 5 to 200 part of a plasticizer, and some additional additives. Actually, the tackifying resin is a mixture of resins, as is illustrated in the example (50 parts of a glycerine rosin ester (“FLORAL” 85, a trademark) and 50 parts of a synthetic polyterpene resin (“WINGTACK” 95, a trademark)). This mixture appears to be required to be reasonably compatible with the base adhesive polymer.
From WO 02/057386A2 adhesive compositions are known, comprising:    i. one or more styrenic block copolymers of the general formula A-C-A (1) or (A-C)nX (2), wherein each A independently is a polymer block of an aromatic vinyl compound, a C is a mixed polymer block (B/I) of butadiene (B) and isoprene (I) in a weight ratio B:I in the range of from 30:70 to 70:30, and said polymer block C has a glass transition temperature (Tg) of at most −50 C. (determined according to ASTM E 1356-98) wherein n is an integer equal to or greater than 2 and X is the residue of a coupling agent,    ii. a tackifying resin, having an aromaticity (in relative percentage of aromatic protons as determined by H-NMR) in the range of from 3 to 18% and preferably from 4 to 14%,    iii. one or more plasticizers.
It will be appreciated that the aromatic modified tackifying resin was taught to be selected from a very specific group in combination with said block copolymers.
Mixtures of S-B-S and S-I-S type block copolymers as suggested in the art do not provide an acceptable alternative either.
Therefore, there is still a need for an adhesive composition in which (1) the adhesive properties are at least equal to those based on S-I-S block copolymers; (2) that shows an improved heat stability in comparison to a composition derived from S-I-S and/or S-B-S block copolymers; and (3) which can be formulated using a single hydrocarbon resin tackifier resin that is readily available and inexpensive.